net-sim: asynchronous exception handling of accept call

When we accept a connection we were using `bracketOnError`.  However we
should not unmask exceptions when executing the in-between callback.
The non-error case is non-blocking, interruptibly masked exceptions
ensures that the simulation state is updated.  In the error case, we
need to uninterruptible mask exceptions to close the channel (since it
is a blocking operation), and then update sim-state which is
a non-blocking operation: for that interruptibly masked exceptions is
enough.

ouroboros-network-framework/src/Simulation/Network/Snocket.hs

@@ -1037,7 +1037,7 @@ mkSnocket state tr = Snocket { getLocalAddr
                                   (TestAddress addr)
         accept_ time deltaAndIOErrType = Accept $ do
             ctime <- getMonotonicTime
-            bracketOnError
+            bracketOnErrorMasked
               (atomically $ do
                 fd <- readTVar fdVar
                 case fd of
@@ -1316,6 +1316,18 @@ mkSnocket state tr = Snocket { getLocalAddr
 -- Utils
 --
 
+-- | Unlike 'bracketOnError' it does not unmask exceptions for the in between
+-- action.
+bracketOnErrorMasked :: MonadMask m
+                     => m a
+                     -> (a -> m b)
+                     -> (a -> m c)
+                     -> m c
+bracketOnErrorMasked before after thing = mask_ $ do
+  a <- before
+  thing a `onException` after a
+
+
 hush :: Either a b -> Maybe b
 hush Left {}   = Nothing
 hush (Right a) = Just a